ELECTRONICS: FUN AND FUNDAMENTALS
By Charles Platt, Author of Make: Electronics
Improbable Slots Carnival Game
Customize it and calculate your odds
of winning the big teddy.
AT A COUNTY FAIR LAST SUMMER,
I saw an interesting new way to lose money.
Players placed 4 quarters among 16 slots
in a board, putting one coin in each row, as
shown in Figure A. Each coin made an electrical connection between 2 contacts at the
bottom of its slot. If you managed to arrange
the coins in a secret winning combination,
the coins completed a circuit, a light came on,
and you won a monster-sized teddy bear.
While I was watching people play, I saw
someone win. After she collected her prize,
she decided to play again — and found that
the previous winning combination didn’t work
anymore. The owner had secretly changed it,
probably with some wiring like the schematic
in Figure B, where a rotary switch can select
4 different preset combinations. (To learn
more about rotary switches, see page 160.)
It seemed to me that constructing a replica
of the game would be easy enough, although
I wanted a more precise way to make contact
with the coins. I cut pairs of brackets from a
length of aluminum angle, ½" by
which I found at Home Depot. I used ½" #3
machine bolts to attach each bracket to a
piece of plastic, although plywood would be
just as good.
Because I wanted some flexibility, I sandwiched a thin piece of rubber under each
bracket, and didn’t screw them down too
tightly. Inserting tap washers would produce
the same effect. Underneath, I clamped #20
solid-conductor wire around each bolt.
If you use this system, drill ample-sized
holes so that you can nudge the brackets into
exactly the right position. They should grip
MATERIALS AND TOOLS
Plywood, ¼"; or plastic sheet,
Aluminum angle, ½" legs ×
5" thickness, at least
a 2' length available at Home Depot and hardware
Thin rubber sheet or tap washers
Machine screws, #3 or #4 × ½", with nuts and
washers ( 32)
Hookup wire, 20 gauge or similar
Battery snap connector, 9V
Rotary switch, single pole, with at least 4 positions
Soldering iron and solder A low-wattage iron will be
insufficient to attach the wires to the screws. Use a
30-watt iron or higher.
a coin tightly when someone pushes it in.
Figure C shows one row of 4 slots.
I wondered if the game would be more
interesting with a different number of slots
or coins. How would this affect the probability
Calculating the odds at the county fair
was easy. There were 4 ways to put a coin in
each row, and 4 rows, so the total number of
combinations was 4× 4× 4× 4 = 256. Therefore,
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